An implantable medical device has a housing containing an atrial and a ventricular sensing stage, a ventricular stimulation pulse generator, a ventricular pacing pulse timer, an atrial tachycardia/fibrillation detector, and a rate determination stage.
The ventricular stimulation pulse generator serves for generation of pacing pulses which can be delivered to a ventricle of a patient's heart via an intracardiac pacing lead connected to the ventricular stimulation pulse generator. To connect such an intracardiac lead to the implantable medical device and to the ventricular stimulation pulse generator generally, a connector in a header of the implantable medical device is provided. The header is part of the hermetically tight housing of the implantable medical device.
Timing of the delivery of pacing pulses is crucial. A wrongly timed pacing pulse can provoke a lethal ventricular fibrillation. This may occur if the pacing pulse is timed to fall into a vulnerable phase of the heart occurring at the end of ventricular repolarization.
A ventriclular depolarisation that is a depolarization of myocardial tissue cells of the ventricle cause a ventricular contraction which is felt as a heart beat. Repolarization leads to a relaxation of the myocardial cells and the expansion of the ventricle. Ventriclular depolarisation causes electrical potentials which can be sensed. A sensed ventricular depolarisation is called a sensed ventricular event indicating a ventricular contraction.
Likewise atrial contractions can be detected by sensing electrical potentials in an atrium of the heart.
Delivery of super-threshold electrical pulses to the myocard, herein referred to pacing pulses, causes a depolarisation of the myocard and thereby a contraction of the heart chamber, e.g. ventricle or atrium, the pacing pulse is delivered to.
Generally, it is a purpose of an implantable medical device such as a pacemaker or defibrillator to treat a malfunctioning heart by delivery of timed pacing pulses or defibrillation shocks to ensure a physiologically adequate pacing rate or to terminate a potentially life threatening fibrillation, respectively.
To determine such a physiologically adequate pacing rate, the rate determination stage is provided. In a common rate adaptive pacemaker, the rate determination stage is connected to a physiological sensor to determine the hemodynamic demand of a patient and to adjust the pacing rate accordingly.
Connected to the rate determination stage is a ventricular pacing pulse timer which triggers a ventricular stimulation pulse generator to deliver a ventricular stimulation pulse at the expiration of a ventricular escape interval (VEI) unless a natural (intrinsic) ventricular contraction is sensed prior to expiration of the VEI. Sensing of a ventricular contraction Vs prior to expiration inhibits the delivery of a scheduled ventricular pace Vp.
The duration of the ventricular escape interval depends on the mode of operation of the pacemaker and on the pacing which, in a rate responsive pacemaker, is determined by evaluating hemodynamic demand specific signal from the physiological sensor.
When a heart is stimulated with a desired pacing rate in an atrium synchronous (atrial synchronized) pacing mode like the DDD-mode, the ventricular escape interval is triggered by an atrial event such as a sensed natural atrial contraction (As) or an atrial pace (Ap, atrial pacing pulse). In a dual chamber pacemaker, a scheduled atrial pacing pulse is delivered if no atrial contraction is sensed prior to the scheduled time of atrial pace delivery.
During atrial tachycardia or atrial fibrillation, the natural intrinsic atrial rate generally would lead to too high a ventricular rate in an atrial synchronized pacing mode. In such case, mode-switching is provided to change the pacing mode to a non-synchronous mode like DDI or VVI. If the pacemaker is operated in a non-synchronous mode of pacing, the VEI usually is triggered by a ventricular event. Since the VEI schedules a ventricular pace which will be triggered if the VEI expires without sensing a ventricular event, in the non-synchronous pacing mode the VEI is the reciprocal value of the stimulation rate.
Regarding this description, the terms “atrial synchronized mode” and “atrium synchronous mode” are used synonymously. Likewise, the terms VV-interval and RR-interval are used synonymously to designate the time interval between consecutive ventricular contractions (sensed as R-waves), the time interval being the reciprocal value of the heart rate.
Atrial fibrillation (AF) represents the most common sustained cardiac arrhythmia in clinical practice, and is associated with increased morbidity and mortality (Kannel et al., 1982; Feinberg et al., 1995). In the absence of advanced or complete heart block, the ventricular rhythm during AF is usually irregular and random. Converging evidence suggested that irregular ventricular rhythm during AF contributes significantly to the symptoms and hemodynamic deterioration that are independent of rapid heart rate (Naito et al., 1983; Daoud et al., 1996; Clark et al., 1997). Recent study further suggested that irregular rather than rapid ventricular response during AF might be responsible for the increased risk of recurrent ventricular arrhythmias in ICD recipients (Gronefeld et al., 2000). In addition, irregular ventricular response during AF may significantly increase the sympathetic nerve activity, which is detrimental in patients with congestive heart failure (Wasmund et al., 2003). Antiarrhythmic drugs have been the mainstay of therapy for the management of patients with AF (Prystowsky et al., 1996). However, drug therapy is often unsatisfactory due to limited efficacy (Pacifico et al., 1999), ventricular proarrhythmia (Prystowsky, 1996), and other undesirable side effects.
Various ventricular pacing protocols have been proposed for ventricular rate smoothing (VRS) and to avoid irreversible AV nodal ablation. Wittkampf et al. (1986; 1988) proposed a VVI pacing algorithm in which the ventricular escape interval (VEI) was increased after each paced beat and was decreased after each sensed beat. This method intended to overdrive spontaneous ventricular event and resulted in more than 93% ventricular pacing. Lau et al. (1990) proposed another pacing protocol in which a ventricular pace was delivered after each sensed R wave. However, the method may result in very high ventricular rate and has potential proarrhythmic effect.
U.S. Pat. No. 5,480,413 issued to Greenhut et al. disclosed a VRS algorithm in which the VEI is dynamically adjusted based on the irregularity measurement of the previous RR intervals. This method cannot prevent sudden ventricular pause after trains of short and regular RR intervals. In addition, regularity control without considering trend of RR intervals may result in undesired high rate or low rate. For instance, gradual but regular increase of RR intervals will lead to further decrease of pacing rate, whereas gradual but irregular decrease of RR intervals will lead to further increase of pacing rate.
U.S. Pat. No. 5,792,193 issued to Stoop disclosed a VRS algorithm in which the VEI is determined according to a flywheel rate which is incremented after a ventricular sense and is decremented after a ventricular pace. The flywheel rate tracks the average ventricular rate, and prevents sudden increase of RR interval. However, depending on the parameter settings of increment and decrement, this method may result in inappropriately fast rate (see U.S. Pat. No. 6,434,424), which may increase the risk of pacing-induced heart failure (Simpson et al., 2001).
U.S. Pat. No. 5,893,882 issued to Peterson et al. disclosed a VRS algorithm in which the VEI is modulated based upon preceding RR interval such that the VEI is set equal to the preceding intrinsic or paced interval, with an increment if the preceding interval is less than the preset target interval, or with a decrement if the preceding interval is greater than the target interval. The performance of this algorithm relies on properness of the target interval setting. In addition, the slope of VEI increment or decrement is fixed, independent of the preceding RR interval and the target interval.
U.S. Pat. No. 6,434,424 issued to Igel et al. disclosed a VRS algorithm in which the VEI is continuously modulated by preceding ventricular event sequence, the stability of the intrinsic ventricular rate, and any atrial pace events. This approach results in greater than 50% of ventricular pacing regardless the rate or regularity of the otherwise intrinsic ventricular rhythm. The increment or decrement of VEI is also fixed, independent of the ventricular rate, regularity, and the event sequence.
U.S. Pat. No. 6,501,988 issued to Kramer et al. disclosed a VRS protocol that allows biventricular sensing in order to avoid fusing beat especially when left ventricle is paced. After a ventricular sense, the VEI moves toward the present RR interval multiplied by a scaling factor at a rate determined by a weighting coefficient. After a ventricular pace, the VEI is increased in an exponential manner up to the basic interval. Such a protocol may result in consecutively high rate ventricular pacing after sensed ventricular event with very short RR interval.
Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to one of skill in the art, through comparison of such systems and methods with the present invention as set forth in the remainder of the present application with reference to the drawings.